Despite the recent interest in adopting HTML5's video tag, there is still one major problem: there is no mandated standard video codec for the video tag. The two main contestants are the proprietary and patended h264, and the open and free Theora. In a comment on an LWN.net article about this problematic situation, LWN reader Trelane posted an email exchange he had with MPEG-LA, which should further cement Theora as the obvious choice.

Unfortunately the choice has already made by Google and Vimeo. h264 will be the codec of choice for maybe 95% of watched videos for years to come. And, most importantly, because of this (partly) it will be the best codec to accelerate in hardware.

Really hope Theora will be hardware accelerated well enough in the future.

All the videos YouTube offers in HTML5 were already encoded in H.264 for flash. So, testing the Video tag functionality on YouTube beta doesn't require re-encoding all if the videos on YouTube, a massive effort. I believe that once Google gets all the bugs worked out of their implementation, they will switch to a more open codec.